Mathematically, the sum of internal angles of triangle is 180, and the sum of internal angles of quadrilateral is 360. And so on, add an edge, and the sum of internal angles will add 180.

The formula of tennis is (n-2) × 180 regular polygon, and the degree of the inner angle is (n-2) ×180 ÷ n.

For example, the sum of the internal angles of a triangle is the sum of three angles inside the triangle, and the internal angle is any one of them.